The present invention relates to compositions for making polyimide resin-reinforced fabrics and to methods of making such fabrics. More particularly, the invention relates to an improvement of such compositions and methos disclosed in U.S. Pat. No. 3,745,149, issued July 10, 1973, and herein incorporated by reference.
The polyimide resins to which the invention relates are noted for their high thermal and oxidative stability, high strength at elevated temperatures, and which exhibit many other outstanding physical and chemical properties useful in high temperature applications.
U.S. Pat. No. 3,528,950 discloses a method for preparing addition-type polyimides, in which an end-capped, low molecular weight prepolymer is prepared in a refluxing solvent. The prepolymer is isolated and then heated to a temperature of 200.degree. to 350.degree. C. to form polyimide macromolecules. The prepolymer has a very limited solubility in organic solvents, and thus does not allow facile impregnation of fibers for fabrication of fiber-reinforced polyimide composite articles.
An improved method for preparing high molecular weight addition-type polyimides is described in U.S. Pat. No. 3,745,149. In this method, the polyimides are synthesized from a mixture of a polyfunctional amine, a polyfunctional ester, and an end-capping agent.
The polyfunctional ester (a) has the formula: ##STR1## wherein R.sub.1 is a tetravalent aryl radical and R.sub.2 is alkyl or hydrogen, at least two R.sub.2 groups being alkyl.
The polyfunctional amine (b) has the formula: EQU H.sub.2 N--R.sub.3 --NH.sub.2
wherein R.sub.3 is a divalent aryl radical.
The end-capping agent (c) has the formula: ##STR2## wherein R.sub.2 is alkyl or hydrogen, at least one R.sub.2 group being alkyl and R.sub.4 is a divalent radical of the formula: ##STR3## wherein R.sub.5 is hydrogen or lower alkyl of one to four carbon atoms, preferably methyl.
The molar ratio of a:b:c being n:n+1:2 wherein n has a value of 1-20, the compounds being present in an amount of 30 to 70% by weight of the solution.
A solution of the foregoing components is used for the preparation of reinforced fibers or fabric. The solution is impregnated into the fibers and then heated at a temperature of up to about 210.degree. C. to remove solvent and to form a prepolymer in the fibers or fabric and the thus prepared "prepreg" material is subsequently heated at a temperature of about 275.degree. to 350.degree. C. to form the polyimide resin in the fibers or fabric. Usually, solvent removal is effected at a temperature of about 50.degree. to 120.degree. C. and prepolymerization is subsequently effected at a temperature of about 125.degree. to 210.degree. C. The prepolymer has a molecular weight of about 400 to 10,000 and the polyimide resin has a molecular weight of over 10,000. Further details concerning the composition and its use will be found in the patent disclosure.
One of the advantages of the composition and method described in U.S. Pat. No. 3,745,149 is that there is no need for prior synthesis of the polyimide procuesors. However, one limitation of the method is that impregnated fiber materials, more commonly known as prepreg, prepared in accordance with the monomer solutions described in the method exhibit limited tack "as prepared" and limited retention of tack when exposed to ambient conditions. Tack, and particularly tack retention, is a desirable property of the prepreg material and it is a disadvantage of the compositions of U.S. Pat. No. 3,745,149 that tack retention is poor.
It is an object of the present invention to provide a novel composition and method for forming polyimide resin-reinforced fibers or fabric. It is a further object to provide such compositions wherein, at a "prepreg" state, the prepreg material exhibits good tack and tack retention properties.